Backlight issues are very common in MacBooks, where the screen will be extremely dim and will not light up, even when changing the brightness. MacBook Airs in particular commonly develop this issue, due to the area of the backlight circuit on the logic board, and a potential of dust buildup becoming damp, gradually damaging components.

If your device has this issue, you can usually see a dark image by shining a bright light through the back Apple logo or onto the screen.

Backlight problems can be caused by a number of different things; but it is usually either the logic board (most likely), LCD cable or the screen. Due to this, it is difficult to say for certain what is required without looking at the device first.